Methods of banking and piling have been conventionally employed to turn such weak soil layer in rivers and harbors into a stabilized foundation ground necessary for civil construction such as roads, banks, and factory sites by stabilization treatment of said weak soil layers. In these cases, the banking construction method is oridinarily employed for the purpose. However, a great deal of sand and many days of work are required for reclamation. On the other hand, with the piling method, some piles must be driven in through the weak soil layer until they reach a rock ground base. Thereafter, plate materials, nets, mats and/or sheets, on which banking is carried out and structures are built, are arranged and provided on and around said piles. Though said piling method appears simple, it requires highly-specialized knowledge and skills, and engineers with many years experience, when said piling work is carried out at all necessary points by use of piling machines, if said weak soil layer covers a wide area as may be the case in lakes, marshes, harbors, and the sea. Furthermore, it is unavoidable that said piling work costs a great deal and requires many days of construction work, thereby causing long-term delays.